...I just went all the way back to v10.2.7. Yeah. That's bad. Like really bad. I have no idea what we were thinking
You can however work around it. Instead of setting the post build, append to it:
ie
<PostBuildEvent>
$(PostBuildEvent)
REM My other postbuild event stuff here
</PostBuildEvent>
Or declare your own post-build event target (which is what the 10.2.x package probably should have done):
<Target Name="MyPostBuildEvent" AfterTargets="Build">
<PropertyGroup>
<MyPostBuildEventCommand>
REM Run my script here
</MyPostBuildEventCommand>
</PropertyGroup>
<Exec Command="$(MyPostBuildEventCommand)" />
</Target>